It is well known to employ rotary actuators for positioning magnetic heads of a magnetic disk file over selected information bearing tracks on the disks. The total angular range of movement necessary to move the heads across all the tracks is typically of the order of fifteen degrees. To minimize the time to access a given track, it is desirable that the actuator should have a high torque/inertia ratio. Because the movement has to be controlled with great precision by a servo system, it is also desirable that the actuator should have a very linear response to drive current, and that the structure should be free of resonant modes which could affect the stability of the servo system.
A state of the art rotary actuator for this application is described in UK Pat. No. 1,342,495, and in an article by J. S. Heath in the IBM Journal of Research and Development (July 1976, pages 389 to 397). This actuator is of the so-called swinging arm type which comprises a voice coil on one end of a pivotally mounted head/arm assembly. When energized, the voice coil moves, within a magnetic flux gap, in an arc to which its axis is always tangential, to cause rotation of the head/arm assembly about the pivot. This type of actuator does not provide balanced torque about the pivot, leading to reactions in the pivot bearings which can cause resonant vibration in the actuator body. These vibrations can produce misregistration between heads and tracks or cause undesirable resonances in the servo system. Nor is the torque/inertia ratio of the swinging arm actuator as high as it could be since part of the moving coil, at one extremity of the moving mass, is not active in producing torque. It is also difficult to provide effective magnetic shielding of the swinging arm actuator, which is important in magnetic information storage applications such as a disk file.
Torque balance can be achieved in a truly rotary actuator, more like a conventional electric motor, in which the rotor is coaxial with the axis of rotation of the head/arm assembly or other load.
One such rotary actuator is described in a publication "Limited Rotation Motor" by P. Y. Hu (IBM Technical Disclosure Bulletin, Volume 16, Number 7, December 1973, page 2259). This publication describes a way of converting various known types of d.c. motors to limited rotation devices, and shows one motor with a tubular printed circuit armature having longitudinal strip windings over the entire surface of a former. Power is supplied through flexible straps to fixed taps on the armature so that a fixed path of current flow is created. The path is such that the current interacts with a stator magnetic field to cause the limited rotation. Because of the need for a former, this type of motor does not maximize the current density in the flux gap. Furthermore, the rotor must be considerably longer than the magnetic flux gap length to accommodate the offsetting of the windings at the rotor ends. This increase in length will lower the torsional resonance frequency of the rotor.
Another rotary actuator is described in a publication "Meter Movement Rotary Actuator" by H. G. Dickie and A. R. Hearn (IBM Technical Disclosure Bulletin, Volume 22, Number 1, June 1979, page 355). This actuator resembles a rugged moving coil meter with a broadened coil. The actuator has a cylindrical shell rotor connected coaxially to a torque transmitting output shaft. The rotor wire is wound over the top and bottom of a hollow sided cylindrical former so that there is only wire and no former material in the working magnetic gap. For efficiency and to avoid out of plane forces, the wire must be wound as near to the axis as possible. This is difficult to achieve in practice and leads to a considerable build up of wire above and below the former. The torsional stiffness of the rotating portion of this actuator is not high because there is unsupported thin sheet of wire in the gap and because output torque is transmitted through a relatively thin output shaft. Thus the torsional resonance frequency may be lower than that which can be tolerated by a high performance servo system.
A rotary actuator for an entirely different application, is shown in UK Pat. No. 980,487. This patent shows a "torquer" and a "pickoff" for a gyroscope in which a cylindrical shell rotor has, deposited on a cylindrical former, two diametrically opposed square pancake coils, the axial limbs of which interact with the poles of an internal permanent magnet stator. The torquer restores a gyroscope containing drum to a predetermined orientation fixed by pickoff coils also on the rotor. The torsional rigidity of the torquer is almost entirely that of the former which by its presence in the gap reduces the motor efficiency.